How to Install a Commercial Subpanel

1. Run the load calculation first

Size the panel from an Article 220 load calculation, not a guess. Add up the loads it will serve: lighting per Table 220.42(A) or the actual connected load, whichever is larger (the table lived at 220.12 before the 2023 edition), receptacles per 220.14 with the Table 220.47 demand factors (Table 220.44 in the 2020 edition), motors per 430.24. Continuous loads — on for three hours or more, meaning most commercial lighting — count at 125 percent per 215.2(A)(1) and 215.3.

Then leave room. A panel calculated at 92 percent of its rating on day one is a callback — if the calc lands near the top of a standard size, step up.

2. Size the feeder and its overcurrent device

The feeder OCPD lives at the supply end — a breaker in the upstream panel or distribution board — and per 408.36 the panelboard must be protected at no more than its rating. Simplest practice: 100 A breaker for a 100 A panel, 225 A for a 225 A panel.

Pick conductors from Table 310.16 at the 75°C column for typical terminations, after any ambient or fill adjustments from 310.15. For a 100 A feeder that's 3 AWG copper; for 125 A, 1 AWG copper. Check voltage drop too — the informational note to 215.2(A) suggests keeping feeder drop to 3 percent. Not enforceable, but a long run at full load will make you wish you'd listened.

Size the EGC from Table 250.122 by the feeder OCPD rating: 8 AWG copper for a 100 A device, 6 AWG copper for over 100 A through 200 A. Upsize the phases for voltage drop and 250.122(B) requires upsizing the EGC proportionately.

3. Verify the fault current and AIC rating

Every breaker in the new panel needs an interrupting rating at or above the available fault current at its line terminals — that's 110.9. Get the fault current from the utility or the service documentation, then account for your feeder run; fault current drops with conductor length.

Standard 10 kAIC breakers cover many locations, but a panel close to a large transformer can see well above that. If fault current exceeds the breaker rating, use higher-AIC devices or a series-rated combination per 240.86 — valid only for manufacturer-tested, listed combinations. Note that 110.24 requires the maximum available fault current and calculation date to be field-marked on service equipment (other than dwellings); documenting it at the new panel as well is good practice and commonly spec-required.

4. Locate the panel for working clearance and dedicated space

NEC 110.26(A) sets the minimums: working space at least 30 inches wide (or the equipment width, whichever is greater), 36 inches deep for Condition 1 at 150 V to ground or less, and 6.5 feet of headroom. Depth goes to 42 or 48 inches at higher voltages with grounded or exposed live parts opposite — check Table 110.26(A)(1). Hinged panels must open at least 90 degrees.

Don't forget 110.26(E): the panel's footprint, from the floor to 6 feet above the equipment or to the structural ceiling, is dedicated electrical space — no duct, no plumbing, no foreign systems. Fight for it at rough-in; the mechanical trades will take it if you don't.

5. Mount the panel and run the feeder raceway

Set the enclosure plumb and anchored to structure — strut rack on masonry, or backing in a framed wall. Keep the highest breaker handle at or below 6 feet 7 inches per 240.24(A). Size the raceway per Chapter 9, Table 1 fill (40 percent for three or more conductors).

Pull all four feeder conductors together: three phases, neutral, EGC. Where the building has both voltage systems, identify feeder conductors by phase and system per 215.12(C) (branch circuits fall under 210.5(C)) — commonly brown-orange-yellow at 480Y/277, black-red-blue at 208Y/120 — document the scheme at the panel and keep it consistent building-wide.

6. Terminate: isolated neutral, bonded equipment ground

This is the step that fails inspections. The neutral-to-ground bond happens in one place: the service disconnect (250.24(A)(5)). Downstream, neutral and ground stay separated. The subpanel neutral bar must be isolated from the can — do not install the green bonding screw or strap that ships in the box. Leave it in the bag.

Install a separate equipment ground bar bonded to the enclosure; land the feeder EGC and all branch EGCs there, and the feeder neutral on the isolated bar. Bond the neutral downstream and neutral current will return on the EGC, raceway, and every metallic path in parallel — a shock hazard and an instant red tag.

Torque every termination to the value marked on the equipment, with a calibrated tool, per 110.14(D). Values differ by lug and conductor size — use the label, not memory.

7. Test before you energize

Megger phase-to-phase, phase-to-ground, and neutral-to-ground on the dead feeder with load disconnected. A neutral-to-ground reading near zero ohms means a downstream bond — find it before energizing. Verify torque marks on every lug and that the bonding screw is out.

Energize the feeder breaker with the cover on and no load, verify voltage phase-to-phase and phase-to-neutral at the lugs, then load circuits one at a time. Confirm rotation on three-phase loads before releasing equipment.

8. Fill out the directory and label the panel

NEC 408.4(A) requires every circuit to be legibly identified with enough detail to distinguish it from all others, and the description can't depend on transient occupancy — 'Bob's office' fails, 'receptacles, NW offices 210–214' passes. Type it; handwritten directories age badly.

408.4(B) requires marking where the supply originates — 'Fed from Panel DP-1, Breaker 14, Electrical Rm 101' on the can. Add the available fault-current documentation (the 110.24 field marking itself is required at the service equipment) and any required arc-flash label.

Why can't the neutral be bonded at a subpanel?

The NEC allows the neutral-to-ground bond only at the service disconnect. Bond them again downstream and neutral current flows back on the equipment grounding conductor, raceways, and enclosures in parallel, putting current on things people touch.

Does a commercial subpanel always need a four-wire feeder?

You need the phase conductors, a neutral if the panel serves line-to-neutral loads, and an equipment grounding conductor. The EGC can be a listed metallic raceway in some designs, but a wire-type EGC pulled with the feeder is standard practice and what most specs call for.

What size ground wire does a 100 A subpanel feeder need?

Per NEC Table 250.122, a feeder protected at 100 amps requires an 8 AWG copper equipment grounding conductor. If the phase conductors are upsized for voltage drop, the EGC must be upsized proportionately.

How much clear space does the code require in front of a panel?

NEC 110.26 requires working space at least 30 inches wide, 36 inches deep for the most common condition, and 6.5 feet of headroom. The footprint above the panel up to 6 feet or the structural ceiling is dedicated to electrical equipment only.

What does the AIC rating on a breaker mean?

AIC is the ampere interrupting capacity — the maximum fault current the breaker can safely interrupt. Every breaker must be rated at or above the available fault current at its terminals, so know that number before ordering breakers.

Do I need a permit to add a subpanel in a commercial building?

Almost certainly yes. Adding a panelboard is permit work in virtually every jurisdiction and gets inspected under the locally adopted NEC edition. Check with your building department first.